Rainbow Key

The Rainbow Key is one of the high-end currency Items in Mobius Final Fantasy.

Rainbow Keys are used to purchase "Core" materials from The Rift regions. These are used to unlock advanced Areas that can reward Extreme Sicarius cards, which have abilities equal in power to a Supreme Card, but without being at the mercy of the rare card summon rate.

Acquisition
Rainbow Keys are used as a high-end reward for veteran players. They currently are obtained in only two ways:

Ranked events
Ranked "Tower" events offer at least one for every player who reaches the top 2,000 scores. Doing so usually requires a player to complete at least 3-5 full Phantasmic Loops. (Boss Kills > ~50)

Other events

 * The 3rd Anniversary Campaign offered 24 Rainbow Keys as a reward to players who have purchased at least 24 Mobius Gift Boxes.
 * The two Greater Weapon Summon Batches offered 4 Rainbow Keys as one of the eight random rewards for each batch.

Use
Each region within The Rift has two areas marked with a prismatic "teleport station" and a treasure chest. If you have enough Rainbow Keys, the treasure chest will change to red/gold. Tap the treasure chest to exchange your Key(s) for that area's Cores:

You can then use the Cores to unlock higher difficulty areas in the region, up to the top "Ultimate Rift" area that rewards an Extreme Sicarius card.